The present invention relates to an aqueous starch slurry adhesive. In particular, the present invention relates to an aqueous starch slurry adhesive, which can be produced at a single step by dispersion in cold water followed by stirring.
Starch has been used in an adhesive for paper and paper board. In order to provide strong adhesion within a short period of time in the manufacture of paper board, a Stein-hall method (hereinafter referred to as SH method) has been widely employed. In this method, a main, which is an aqueous dispersion wherein ungelatinized starch particles are suspended, is added to a carrier which is a completely gelatinized starch aqueous solution. When the adhesive obtained by this method is heated, the ungelatinized starch particles of the main rapidly dissolve in heated water, while simultaneously water evaporates, whereby producing semi-solid material having a high viscosity and providing an initial adhesive power. It is necessary for the carrier to have a sufficient viscosity so that the main particles do not sediment in the carrier and so that the carrier is suitably applied to paper or the like.
However, in the SH method, the adhesive becomes highly viscous even with the presence of a small amount of starch. Therefore, it is very difficult to increase the concentration of the carrier starch to 15% or higher. Therefore, the water content becomes higher, which leads to problems such as warp. There is proposed a technique of using decomposed starch such as dextrin in order to increase the concentration. However, this technique poses a problem like decrease in adhesive strength.
As is typically disclosed in Japanese Patent Un-examined application (hereinafter referred to `J. P. Kokai`) Sho No. 52-127938, there is proposed a technique of using, as a carrier, processed starch which is decomposed or modified to a suitable degree, in order to increase the concentration. However, the total starch content of the main and carrier is at most 30% and, therefore, high concentration can not be obtained.
An attempt has been made to increase the amount of the ungelatinized starch of the main so as to highly increase the starch concentration as a whole. However, this technique has the following problem. Namely, in order to obtain the initial adhesion power, it is necessary for the main to dissolve by the heat at the time of adhesion and the immediately gels. In addition, it is indispensable to dissolve borax into the carrier in order to provide the starch adhesive with a high speed adhesive property. It is general that in the SH method, the carrier does not immediately gel by itself because the starch concentration is low, even though borax is present in the starch solution of the carrier. However, in case where the starch concentration of the main is raised and where borax is present in an amount sufficient for causing gelatinization of the carrier, the main dissolves and the carrier gels, which leads to increase in viscosity. In other words, it is difficult to increase the total starch concentration even by increasing the amount of the ungelatinized starch in the main, so long as the carrier is a starch solution.
Further, the SH method requires two steps, namely, the first step of gelatinizing starch by heat to form a carrier and the second step of adding an aqueous dispersion of a main to the resultant carrier. Therefore, this method is complicate. Accordingly, there has been proposed a method of using gelatinized cereal powder for the carrier. In this method, both the gelatinized cereal powder and ungelatinized starch or ungelatinized cereal powder are dispersed into cold water, to dissolve only a portion soluble in cold water (gelatinized starch) so as to form a carrier (J. P. Kokai Sho No. 50-63023). In this method, it becomes unnecessary to form a carrier by heating. However, the high concentration cannot be obtained and the viscosity is unstable, because the carrier is made of gelatinized starch.